The present invention relates to gear shift control systems for automatic transmissions, and in particular to such a system wherein the rate of increase in air quantity taken into the engine is detected to effect downshift operation.
Conventional automatic transmissions for automotive vehicles are provided with gear shift control valves which operate on a balance between a speed-sensitive governor oil pressure and a throttle pressure to supply working fluids for application and release of various clutches and bands. For a given throttle opening, an increase in the governor oil pressure will automatically result in a reduction in gear ratio (upshift operation). A downshift valve is also provided to supply pressure to act against the governor oil pressure to change to a higher gear ratio (downshift operation) when the vehicle drive attempts to obtain a rapid acceleration. However, the downshift valve is so adjusted that it operates only when the throttle opening exceeds a predetermined value. Therefore, when the downshift valve operates the engine has developed a greater amount of torque than is required due to the wide open throttle, thus resulting in a greater amount of acceleration than the driver desires to obtain.
To overcome this problem prior attempts have involved the use of a detector for sensing the rate of increase in throttle opening or the rate of decrease in intake vacuum pressure as a controlling factor to effect downshift operation, as disclosed in Japanese Pat. No. 53-11623.
However, it is found that either the throttle opening or intake vacuum is not proportional to the engine torque over the full range of such operating parameters, so that an optimum amount of acceleration is difficult to achieve.